Studies of the transmural distribution of myocardial blood flow will be carried out in chronically instrumented awake dogs using 15 micron diameter radionuclidelabelled microspheres injected into the left atrium. To test the hypothesis that calcium entry blocking drugs enhance subendocardial perfusion in the presence of a proximal flow-limiting coronary stenosis, the effects of one such drug, diltiazem, on the transmural distribution of myocardial blood flow will be examined in dogs with a proximal coronary artery stenosis. The effects of treadmill exercise on perfusion of collateral-dependent myocardium will be studied in chronically instrumented dogs one month after gradual occlusion of the left circumflex coronary artery with an Ameroid occluder. This study will characterize the behavior of the developing collateral system at rest and during exercise. The effect of the calcium entry blocker, nifedipine, on the volume and distribution of collateral blood flow during exercise will be examined in dogs with chronic coronary artery occlusion. To test the hypothesis that coronary artery stenosis resistance may be influenced by the intra-arterial distending pressure at the site of the stenosis, coronary stenosis resistance will be examined in chronically instrumented dogs at rest and during treadmill exercise when vasodilation of the coronary resistance vessels results in a reduction of intra-arterial pressure distal to the stenosis.